1. Field of the Invention
The present invention relates to a reversible thermosensitive coloring composition comprising an electron-donating compound and an electron-accepting compound, capable of inducing color formation using a coloring reaction between the electron-donating compound and the electron-accepting compound. The present invention also relates to a reversible thermosensitive recording medium comprising the reversible thermosensitive coloring composition, which recording medium is capable of reversibly forming a colored image therein and erasing the same therefrom by controlling thermal energy applied thereto.
2. Discussion of Background
There is conventionally known a thermosensitive recording medium which uses the coloring reaction between an electron-donating compound (hereinafter referred to as a coloring agent) and an electron-accepting compound (hereinafter referred to as a color developer). This kind of thermosensitive recording medium is widely used, for example, for facsimile apparatus, word processors, and printers for use with instruments for performing scientific measurements.
However, the coloring reaction of a conventional thermosensitive recording medium of this type currently used in practice has no reversibility, so that color development and decolorization cannot be alternately repeated.
Among published patents, there are several proposals for a thermosensitive recording medium which can reversibly carry out the color development and decolorization using a coloring reaction between a coloring agent and a color developer. For example, a thermosensitive recording medium using phloroglucinol and gallic acid in combination as color developers is disclosed in Japanese Laid-Open Patent Application 60-193691.
In Japanese Laid-Open Patent Application 61-237684, there is disclosed a reversible thermosensitive recording medium which employs as a color developer a compound such as phenolphthalein or thymolphthalein.
In Japanese Laid-Open Patent Applications 62-138556, 62-138568 and 62-140881, there are disclosed reversible thermosensitive recording media, each comprising a recording layer which contains a homogeneously dissolved composition of a coloring agent, a color developer and a carboxylic acid ester.
Furthermore, in Japanese Laid-Open Patent Application 63-173684, there is disclosed a reversible thermosensitive recording medium which comprises as a color developer an ascorbic acid derivative.
In addition, in Japanese Laid-Open Patent Applications 2-188293 and 2-188294, there is disclosed a reversible thermosensitive recording medium which comprises as a color developer a salt of bis(hydroxy-phenyl)acetic acid or gallic acid, and a higher aliphatic amine.
In the above-mentioned conventional reversible thermosensitive recording media, however, the requirements that the colored image be stable and that the decolorization of the colored image be easy are not always satisfied at the same time, and there are still unsolved problems with respect to the density of the colored image, and also with respect to the stability of the repetition of color development and decolorization operations. Therefore, the conventional reversible recording media are not satisfactory for practical use.
The inventors of the present invention have previously proposed in Japanese Laid-Open Patent Application 5-124360 a reversible thermosensitive coloring composition which comprises as a color developer an organic phosphoric acid compound, an aliphatic carboxylic acid compound or a phenolic compound, each having a long-chain aliphatic hydrocarbon group therein, and as a coloring agent a leuco dye. When such a reversible thermosensitive coloring composition is used, color development and decolorization can be easily carried out by controlling the heating and cooling conditions for the coloring composition, and further, the color-developed state and the decolorized state can be stably maintained at room temperature, and the color development and the decolorization can be reversibly repeated.
In the above-mentioned application, there is also proposed a reversible thermosensitive recording medium which comprises a recording layer containing the above-mentioned reversible thermosensitive coloring composition.
This kind of reversible thermosensitive recording medium attains a practically usable level with respect to the balance between the stability and the ease of decolorization of a colored image, and also with respect to the density of a colored image. However, there is yet much room for further improvement of the coloring composition with respect to the color development and decolorization properties thereof and the broadening of the range of the operating conditions under which the coloring composition can be used.
In Japanese Laid-Open Patent Application 6-210954, a phenolic compound having a specific structure including a long-chain aliphatic hydrocarbon group is proposed as a color developer. However, a reversible thermosensitive recording medium comprising the above-mentioned phenolic compound has the same drawbacks as mentioned above.